Recent research progress of relaxation performances of defects in ZnO-Bi2O3 varistor ceamics

ZnO varistor ceramics have been widely applied to surge absorption and over-voltage protection in electronic circuit and power system because of their excellent non-ohmic characteristics.Therefore,the reaserch on ZnO varistor ceramic has long been a subject of interest for scholars and industrial circles.At present,the conductance theory of ZnO varistor ceramic has been widely studied and reviewed,and several models such as space charge limited current model,NordheimFowler tunneling current model,and Schottky barrier model have been proposed to describe the electronic transmission process and explain the non-ohmic behavior of ZnO ceramic varistor.However,the relationships of the defect structure and defect relaxation with the electrical property of ZnO varistor ceramic remain unclear,which becomes a challenge to developing new ZnO varistor ceramics.In this paper,comments on defect structures and defect types of ZnO ceramics are given,and the theortical calculation of the intrinsic point defects is discussed.Besides,the characterization technologies of the defect relaxations are introduced.The results show that the dielectric loss spectra are widely used to describe the relaxation of ZnO ceramic varistor,especially the spectra in the low frequency can provide more information about defect relaxation of ZnO ceramic varistor.It is also found that the frequency spectra of admittance in a wide temperature range and the temperature spectra of admittance in a wide frequency range play an equivalent role in characterizing the defect relaxation of ZnO ceramic varistor.The thermally stimulated current is considered to be an effective method to verify the relaxation polarization mechanism of the defects.The deep level transient spectroscopy can characterize the intrinsic and extrinsic defect relaxation processes.Moreover,several theories of relaxation mechanisms such as the Cole-Cole theory,Havriliak-Negami theory and Cole-Davidson theory are proposed to analyze the relaxation phenomena of ZnO ceramic varistors.It is suggested that the electric modulus spectrum combined with Cole-Davidson theory is more effective to characterize the defect relaxations in a wide temperature range.From the electrical degradation results, it is found that the extrinsic defect relaxation at grain boundary interface is closely related to the electrical property of ZnO ceramic varistor.A circuit model is also obtained to establish the correlation between defect relaxation and electrical performance of ZnO ceramic varistor.Therefore,the review on defect relaxations may offer some new ideas to optimize the electrical properties of ZnO ceramic varistors by modifying the defect structures.